Delirium, an acute state of diminished cognition and attention, is common in elderly patients. Prior studies demonstrate an association between delirium and increased mortality, length of hospitalization, and rates of discharge to an extended stay facility. While the prevalence of delirium among hospitalized elderly patients is high, ranging from 25 to 60%, the prevalence of delirium among elderly Emergency Department (ED) patients is much lower, approximately 10% of elderly ED patients. This suggests that a large proportion of delirious inpatients develop delirium while in the hospital, as opposed to presenting with delirium. Thus, there is opportunity to identify ED patients at risk of developing delirium, and targeting interventions towards this vulnerable population in an effort to reduce the morbidity from in-hospital delirium. The overall goal of this study is to characterize the subgroup of Emergency Department patients who develop hospital acquired delirium, such that this group may be targeted for delirium prevention efforts in the future. This grant proposes an ancillary study to an ongoing prospective ED-based cohort study that evaluates elderly ED patients for delirium utilizing the Confusion Assessment Method. The parent study focuses on evaluating patients for delirium while in the ED, but does not assess for the development of delirium after admission. In the proposed ancillary study, we will review the inpatient records of patients who were not delirious in the ED and were admitted to the hospital. Utilizing a validated chart-based method, we will determine which of these patients developed delirium while hospitalized. For our first specific aim, we will assess ED-based covariates, including demographics, medical information, and data from the ED course, in the prediction of hospital-acquired delirium. Utilizing multivariable regression analysis, we will develop a profile of elderly patients at increased risk for delirium after admission. In our second specific aim, we will determine whether there are differences in resource utilization and outcomes among patients who present to the ED with delirium, patients who develop delirium while hospitalized, and patients who do not develop delirium at all. Finally, the third specific aim will explore whether biomarkers hold diagnostic potential in identifying which patients will develop delirium after admission. By developing tools that can successfully predict patients who are at high risk of hospital-acquired delirium, researchers and clinicians can target this vulnerable population for delirium prevention efforts with the ultimate goal of decreasing morbidity, mortality, and health care costs associated with this common syndrome.